Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions.

The water we drink and use is running short in the world. We all have to learn how to stop wasting our limited water. One of the steps we should take is to find ways of reusing it. Experiments have already been done in this field.

Today in most large cities, fresh water is used only once, then it runs into waste system. But it is possible to pipe the used water to a purifying factory. There it can be filtered and treated with chemicals so that it can be used again, just as it were fresh from a spring.

But even if every large city purified and reused its water, we still would not have enough. Then we could turn to the oceans. All we would have to do to make use of the seawater on earth is to get rid of the salt. This process is called desalination, and it is already in use in many parts of the world.

The word "limited" in the passage probably means _____________.